Relay Contact Welding

[keithkyll]

If you put a relay or contactor on the input of a VFD, you will have contact welding. Main cause is the capacitors.
If you put a relay or contactor on the output of a VFD, you run the risk of blowing the output devices in the VFD.
Use the safety controls built into the VFD. That's what they're there for.
A relay only has one set of contacts per pole. They should never be used for motor loads. Short life and welding is a guarantee.
Suppressors will help, but won't solve the problem.

A contactor has two contacts per pole, and they are floating. When one side welds, the other side still moves, breaking the weld on the first contact.

My first choice is to redesign, using VFD inputs for control. Secondary safety would be a contactor on the output, with the VFD driving it.

If you must stick with original design, then they must be contactors. You'll never find a relay that won't weld in this application.

Contactor with 120V coil from Automation Direct.

 

[Highland Controls]

Unless there is some kind of timer lockout for the safety circuit, I expect this drive to have a short life. If someone opens and closes a door frequently, the DC charging circuit in the drive gets a workout, and will degrade quickly.

 

[JesperMP]

Using a relay to switch power is a no-no.
Simply use a contactor rated for the VFD or whatever "amplifier" kku is talking about.
A contactor is not much more expensive than a relay. So it is a non-issue.

You can have a contactor on the supply side of a VFD. It must just not be for an application that cause frequent switching of the contactor (which according kku's description it is not).
Depending on the safety of what the VFD drives, it may not be enough though to switch off the contactor. The risk assessment should clarify that.

 

[jparale]

The best way I think is to use a solid state relay. This is an electronic type relay capable of handling inrush current and also the left over induced current from the motor winding.

Using normal elctromechanical relay, you might want considering using RC filter across the relay coil if it is ac or a flywheel diode ( reverse bias cofig) when you're using a dc coil relay.